net offloading: Accept NETIF_F_HW_CSUM for all protocols.

We currently only have software fallback for one type of checksum: the
TCP/UDP one's complement.  This means that a protocol that uses hardware
offloading for a different type of checksum (FCoE, SCTP) must directly
check the device's features and do the right thing ahead of time.  By
the time we get to dev_can_checksum(), we're only deciding whether to
apply the one algorithm in software or hardware.  NETIF_F_HW_CSUM has the
same capabilities as the software version, so we should always use it if
present.  The primary advantage of this is multiply tagged vlans can use
hardware checksumming.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>